Problem: The scale on a map is 7cm : 7km. If the distance between two cities is 28km, how far apart in cm are the two cities on the map?
The scale means that every 7cm on the map corresponds to an actual distance of 7km. An actual distance of 28km is the same as 4 $\cdot$ 7km. The distance between the two cities on the map is 4 $\cdot$ 7cm, or 28cm.